Aside from the obvious comeback of "the Android kernel is Linux"...
The reason NASA wants to use commercial phones is because it wants to use off-the-shelf hardware. That provides it with an already-extant platform with mature drivers, which doesn't need much custom engineering.
I'm not aware of any mature, free-as-in-speech Linux phones with the sort of power and sensor arrays of the Galaxy Nexus. I guess NASA isn't either. Are you?
